CCD imagers are currently used in many applications, including: commercial digital photography, satellite photography, x-ray machines, and other imaging devices. A CCD imager makes use of CCD focal plane(s) and peripheral drive circuitry to form a sensor (or camera).
CCD based sensors utilize the photoelectric effect to produce electric charge signals that are monotonically dependent on wavelength and photon intensity to which the sensor is exposed. The combination of sensor material and optical filters determines the operating bands within which the sensor will generate electrical signals in response to absorbed photons from the scene. The CCD focal plane is divided into pixels and within each pixel electrical charge is photogenerated. Collectively, photogenerated charges in the pixels represent the image data.
A CCD focal plane includes an advantageous property that is absent from photodiode-based focal plane sensors. A photodiode stores signal charges on a capacitor formed by the diode's p-n junctions. A CCD stores signal charges in potential well. This fundamental difference in signal storage can have a tremendous impact when imaging from a moving platform. Specifically, signal charges photogenerated on different photodiodes cannot be efficiently transferred without introducing reset noise. By contrast, signal charges photogenerated in CCD potential wells can be efficiently moved between potential wells without reset noise. Efficient charge movement in CCDs between potential wells can be important for realizing sensors operating from moving platforms.
The issue of image blur should be addressed when imaging from a moving platform. One solution involves using a fast shutter speed, which can avoid blur at the cost of reduced sensitivity. To overcome motion smear issues, CCD sensors are physically and electrically configured to operate in TDI mode, thereby allowing the CCD sensor to capture clear images while the CCD sensor is moving or while the target object is moving. For example, a digital camera on an aircraft or satellite can be configured to take crisp, non-blurry still pictures of the Earth below or something moving over the Earth. As another example, CCD imagers in TDI mode can be used to capture and generate detailed images of articles' barcodes moving at a high velocity down a manufacturing line. TDI mode involves a TDI clock that synchronizes the movement of the CCD sensor and/or target object with the movement of the electrical charges through the CCD wells.